Method for producing magnetic resonance cells

ABSTRACT

A method for making a magnetic resonance cell. Two elongated tubes communicate through orifices with the cell which is surrounded by an oven. One tube contains an ampoule prefilled with a barium film. The other tube has a necked-down portion or seal-off region and an enlarged portion. The enlarged portion has two branches, the first axially aligned with the necked-down portion and the second disposed at an acute angle with respect to the first. A wax carrier is in the first branch and an ampoule containing cesium is in the second. Vacuum pumps evacuate the cell and tubes, after which evacuation the cell is heated. Moving the wax carrier into the necked-down portion of the tube allows the wax to evaporate into the resonance chamber of the cell. After removing the wax carrier from the necked-down portion, one positions the cesiumfilled ampoule in the necked-down portion. The necked-down portion is then heated and sealed to remove the cell and ampoules from the vacuum system. Breaking the ampoules allows the barium to act as a getter and the cesium to escape. The cesium-filled ampoule and necked-down portion may be removed by heating and sealing the necked-down portion at its point of connection to the cell.

United States Patent [1 1 Shernoff et al.

[4 1 Sept. 9, 1975 METHOD FOR PRODUCING MAGNETIC RESONANCE CELLS [73]Assignee: The United States of America as represented by the Secretaryof the Navy, Washington, DC.

[22] Filed: July 16, I974 [21 Appl. No.: 489,002

Related U.S. Application Data [62] Division of Ser. No. 381,834, July 231973, Pat. No.

[52] U.S. Cl 316/20; 316/3 [51] Int. Cl. "01.! 9/385 [58] Field ofSearch 316/17, l8, I9, 20, 27,

[56] References Cited UNITED STATES PATENTS 1,623,323 4/1927 Van Voorhis316/3 l.881,6l6 10/1932 Ives 316/30 X 1,904,895 4/1933 Campbell... 316/62,209,821 7/1941) Lautsch 316/1 3,281,709 10/1966 Dehmclt 324/.5 E3,667,513 6/1972 Della Porta ct a1 316/24 3,675,067 7/1972 Brun 324/.5 F

Primary ExaminerRoy Lake Assistant ExaminerJames W. Davie Attorney,Agent, or FirmR. S. Sciascia; P. Schneider [5 7] ABSTRACT A method formaking a magnetic resonance cell. Two elongated tubes communicatethrough orifices with the cell which is surrounded by an oven. One tubecontains an ampoule prefilled with a barium film. The other tube has anecked-down portion or seal-off region and an enlarged portion. Theenlarged portion has two branches, the first axially aligned with thenecked-down portion and the second disposed at an acute angle withrespect to the first. A wax carrier is in the first branch and anampoule containing cesium is in the second. Vacuum pumps evacuate thecell and tubes, after which evacuation the cell is heated. Moving thewax carrier into the necked-down portion of the tube allows the wax toevaporate into the resonance chamber of the cell. After removing the waxcarrier from the necked-down portion, one positions the cesiumfilledampoule in the necked-down portion. The necked-down portion is thenheated and sealed to remove the cell and ampoules from the vacuumsystem. Breaking the ampoules allows the barium to act as a getter andthe cesium to escape. The cesium-filled ampoule and necked-down portionmay be removed by heating and sealing the necked-down portion at itspoint of connection to the cell.

6 Claims, 4 Drawing Figures PATENTEDSEP 8M5 SFZET 1 [1F 3 METHOD FORPRODUCING MAGNETIC RESONANCE CELLS This is a division of applicationSer. No. 381,834 filed 23 July 1973 now US. Pat. No. 3,860,31 I.

BACKGROUND OF THE INVENTION This invention relates generally tomagnetometers and more specifically to a method for producing a magneticresonance cell which one can employ in a magnetometer as well as variousother devices.

Previous methods for fabricating magnetic resonance cells involved theuse of an inert buffer gas to reduce interaction of the alkali metalatoms with the glass walls of the cell. Often a thin coating of awax-like material was employed to form a lining on the inner surface ofthe glass envelope of the cell to eliminate the interaction between thealkali-metal vapor and the glass. Old methods for applying such acoating are described in the literature regarding the fabrication ofrubidium, magnetic resonance cells.

The disadvantages of the old methods primarily concern maintaining thepurity of the cell. Coating the inner surface of the cell with wax ofteninvolved direct heating of the wax. Over-heating of the wax was a commonoccurrence which resulted in cracking it to produce organiccontamination of the cell. Furthermore. difficulty was encountered inremoving Wax from parts of the manufacturing apparatus. Removal of thewax was necessary to avoid the possibility of it coming into contactwith hot cesium vapor and producing unwanted compounds. Finally ascaled-off cell requires the presence of a gettering agent to maintainthe existence of a high vacuum. Conventional methods use a barium getterring-- barium in a nickel tube" which is flashed to release the bariumthat acts as the gettering agent. However, the presence of nickel, beinga magnetic substance, hinders the performance of the cell.

SUMMARY OF THE INVENTION The present invention provides a method offabricating an alkali-metal. magnetic resonance cell with high signalstrength, minimum interactions between the walls of the cell and theenclosed alkali-metal vapor, minimum sensitivity of resonance-line widthto magnetic-field gradients, and stability of performance over anextended period of time. To coat the inside surface of the cell withwax, one moves the wax into direct and immediate communication with thecell interior, eliminating the possibility of contamination of otherparts of the apparatus and the necessity for flaming various parts ofthe apparatus to remove unwanted wax. The alkali metal can also be movedinto direct and immediate communication with the cell. eliminating theneces sity of heating and, therefore, reducing the possiblity of themetal reacting with the wax. A film of gettering agent is employedrather than a gcttering ring, also reducing the likelihood ofcontamination.

An object of the present invention is to produce a magnetic-resonancecell which is free of impurities.

Another object is to produce a magnetic resonance cell having a highsignal strength with minimum sensitivity of resonance-line width tomagnetic-field gradicnts.

Still another object is to provide a cell which is stable over extendedperiods of time and which has a minimum of interactions between thewalls of the cell and the enclosed alkali-metal vapor.

Other objects, advantages, and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of theapparatus employed to produce the magnetic-resonance cell of the presentinvention;

FIG. 2 is a schematic of a portion of the apparatus shown in FIG. Idepicting the position of the elements during an inital step of themethod;

FIG. 3 is a schematic of the magnetic resonance cell during the finalstep just prior to completion; and

FIG. 4 is a schematic of a prior art device employed to make a magneticresonance cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To gain a better understandingof the present invention, one must first understand the typicalprocedures employed in the past to produce a magnetic resonance cellcontaining an alkali-metal vapor. FIG. 4 illustrates such a prior artdevice.

A glass sphere 72 forms what eventually will become the resonancechamber 70. The sphere 72 is surrounded by a first oven 74. Twoelongated tubes 76 and 78 communicate with the resonance chamber throughorifices 80 and 82. Tube 78 contains a gettering agent, in this case abarium getter ring 84. Tube 76 has two branches 86 and 88 which areperpendicular to the central axis of tube 76. The tubes 86 and 88contain magnets 90 and 92 enclosed in glass. These enclosed magnets areheld in place by magnets (not shown) outside of the tubular extensionsor branches 86 and 88.

The bottom of each tubular extension 86 and 88 is sealed by a thin layerof glass having frangible tips 94 and 98. In the chamber formed in thebottom of tube 86 a wax-like material 96 is placed. In the chamberformed in the base of tubular extension 88 an alkalimetal pellet 100 isplaced. A valve 102 controls communication between a vacuum pumpingsystem 104 and the tubular extension 76. A larger oven I06 encloses thesmaller oven 24 and the tubular extension 76.

The technique employed to produce the cell would typically include thefollowing steps. A vacuum would be obtained in the apparatus by means ofthe pump system I04. Sphere 72 would then be heated to about 400C bymeans of oven 74 for several hours. Valve 102 would then be closed andfrangible tip 94 broken using the enclosed magnet 90 directed by amagnet (not shown) outside branch 86. Oven 74 would be turned off andoven I06 turned on to drive the wax 96 into the resonance chamber 70.After wax 96 has evaporated oven I4 is turned off and the tubulationbetween the orifice 80 and the frangible tip 94 would be gently flamedto remove any wax from the tubulation. Overheating of the wax duringthis step often occurs cracking the wax to produce organic contaminationof the cell. Furthermore, it is also difficult to ensure the completeremoval of the wax from the tubulation to avoid contamination with thealkali metal that is introduced in the next step. After the tubulationhas been flamed, frangible tip 98 is broken using the enclosed magnet92. A hand torch is then used to evaporate the alkali metal until asufficient amount appears in the region between the small orifice 80 andthe seal-off region 77. The seal-off region 77 is then heated to softenthe glass. allow a seal to form, and remove the cell from the vac uumsystem. Finally, the barium getter ring 84 is flashed. As noted abovethis step often results in contamination of the cell.

The present invention overcomes these deficiencies by employing theapparatus depicted in FlGv l. A glass sphere l encloses what eventuallywill become the resonance chamber 12 of the completed cell. Twoelongated tubes 14 and 16 communicate with the cell through orifices l8and 20. Only one oven 13 is employed in the present invention to heatthe sphere l0. Tube 14 contains a glass ampoule 22 which has a frangibletip 24. The ampoule 22 has been prefilled with a gettering film 26,preferably barium. The other tube 16 has a necked-down portion orseal-off region 28 and an enlarged portion having two branches 30 and32. The first branch 30 is actually in line with the necked-down portion28, and the second branch 32 is disposed at an acute angle with respectto the first. A wax carrier 34 is in the first branch 30. The waxcarrier 34 is composed of a glass-enclosed magnet 36 which is moved bymeans of a magnet (not shown) outside the tubular extension. One end 38of the wax carrier 34 is hollowed out and contains a wax 40. The secondbranch 32 also contains a glass-enclosed magnet 44 similar to the oneemployed in branch 30. Above the glass-enclosed magnet 44 is an ampoule46 having a frangivle tip 48 which encloses an alkali metal 50. At theend of the tubular extension 16 is a valve 52 which controls theconnection to a vacuum pumping system 54.

Of course. both of the ampoules 22 and 46 must be prepared prior toforming the apparatus, which can be constructed using standard glassblowing and handling techniques. The gettering film 26 and ampoule 22are produced by flashing a commcrical gettering ring inside the ampoule22 under high vacuum. The metal ring is then removed from the ampoule22, and it is sealed off at the frangible tip 24. Although barium ismost extensively used as a getter material, magnesium. calcium, sodiumand phosphorus may also be employed. Although any alkali metal may beplaced in ampoule 46 cesium is preferred. The wax 40 may be purchased ina highly purified form or purified by vacuum distillation prior to beingplaced in the wax carrier 34. Both hexa triacontane and octacosane havebeen employed as a wax. In addition. the two may also be mixed. Otherlong-chain hydrocarbon. wax-like substances may also be employed.

To form the cell a vacuum is first obtained in the apparatus by means ofthe pump system 54. Sphere is then heated to about 400C by means of theoven 13 for several hours for outgassing purposes. Sphere [0, like otherglass parts of the apparatus. is preferably Pyrex. The oven is thenturned off, and the wax carrier 34 is moved by means ofa magnet (notshown) outside the branch into the seaLoff region 28 as shown in FIG. 2.The temperature inside the oven l3 is then raised to about 200C,depending on the specific wax being used, to drive the wax into theresonance chamber 12. Because the wax carrier 34 is directly andimmediately adjacent to the resonance chamber l2, contaimination ofother parts of the apparatus with the wax is avoided. Next the oven isturned off. and the cell and tubular extensions are removed from theoven. lfdcsired the scal oft region 28 may be gently flamed at thistime. By means of the enclosed magnet 42 the ampoule 46 is moved intothe seal-off region 28 just vacated by the wax carrier 34. Next theseal-off region 28 is heated at 58 with a hand torch to seal the regionand remove the rest of the tubular extension arriving at the structureshown in FIG. 3. Simply shaking the cell breaks the frangible tips 24and 48. Breaking frangible tip 48 allows the alkali metal 50 in theampoule 46 to evaporate into the cell where it remains as a gas. Thebarium film 26 in the ampoule 22, now exposed, can perform its getteringfunction. If desired, the necked-down portion 28 can be removed andsealed off at 60 by use of a torch although it is better to allow thetubes to remain as they are since the alkali and the getter continue tofunction-the alkali to evaporate into the sphere and the getter toabsorb gases.

Thus, a method of accomplishing the above is the following:

1. place an ampoule 22 having a frangible tip and containing getteringmaterial 26, inside a first tube 14 having an open end;

2. place a second ampoule containing wax 40 inside a second tube open atboth ends;

3. place a third ampoule 42 having a frangible tip and containing analkali metal inside a third tube 32 open at only one end;

4. connect the open ends of the second and third tubes to one end of atube (the necked-down tube 28) which is smaller in diameter than the twoto which it connects, the connection being made so that all three tubescommunicate with each other;

5. connect the open ends of the first tube and the necked-down tube toorifices in the sphere 10;

6. connect the open end of the second tube to vacuum pumping means;

7. place the sphere inside an oven;

8. evacuate the system of tubes and sphere;

9. heat the sphere to remove gases from the walls and then allow thesphere to cool;

10. move the ampoule containing the wax inside the necked-down tube to aposition near the Sphere;

11. heat the sphere to a temperature sufficient to melt the wax withoutdegrading it so that the wax coats the inside of the sphere;

12. remove the sphere and tubes from the oven;

13. move the wax-containing ampoule back and out of the necked-downtube;

14. move the alkalimetal-containing ampoule into the necked-down tube;

15. heat the necked-down tube at the end away from the end communicatingwith the sphere to seal it off and break off the second and third tubestherefrom after the seal; and

16. break the frangible tips of the remaining ampoules.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. it is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

l. A method of making a magnetic resonance cell out of a glass spherecomprising the steps of:

placing an ampoule, having a frangible tip and com taining getteringmaterial. inside a first tube having an open end;

placing a second ampoule Containing wax inside a second tube open atboth ends;

placing a third ampoule having a frangible tip and containing an alkalimetal inside a third tube open at only one end;

connecting the open ends of the second and third tubes to one end of anecked-down tube which is smaller in diameter than the two to which itconnects. the connection being made so that all three tubes communicatewith each other;

connecting the open ends of the first tube and the necked-down tube toorifices in said sphere;

connecting the open end of said second tube to vacuum pumping means;

placing said sphere inside an oven;

evacuating the system of tubes and sphere;

heating said sphere to remove gases from the walls and then allowing thesphere to cool;

moving said ampoule containing the wax inside the necked-down tube to aposition near the sphere;

heating the sphere to a temperature sufficient to melt the wax withoutdegrading it so that the wax coats the inside of the sphere;

removing the sphere and tubes from the oven; moving said wax-containingampoule back and out of said necked-down tube; moving saidalkali-metal-eontaining ampoule into said necked-down tube; heating thenecked-down tube at the end away from the end communicating with thesphere to seal it off and breaking off the second and third tubestherefrom after the seal; and breaking the frangible tips of theremaining ampoules. 2. The method of claim 1, wherein the alkali metalis cesium.

3. The method of claim 1, wherein the getter comprises a film inside itscontaining ampoule.

4. The method of claim 1, wherein the getter comprises a film of bariuminside its containing ampoule. 5. The method of claim 1, wherein thegettercontaining and alkali-metal-containing ampoules are made of Pyrexglass.

6. The method of claim 1, wherein the ampoules and the tubes are made ofPyrex glass

1. A method of making a magnetic resonance cell out of a glass spherecomprising the steps of: placing an ampoule, having a frangible tip andcontaining gettering material, inside a first tube having an open end;placing a second ampoule containing wax inside a second tube open atboth ends; placing a third ampoule having a frangible tip and containingan alkali metal inside a third tube open at only one end; connecting theopen ends of the second and third tubes to one end of a necked-down tubewhich is smaller in diameter than the two to which it connects, theconnection being made so that all three tubes communicate with eachother; connecting the open ends of the first tube and the necked-downtube to orifices in said sphere; connecting the open end of said secondtube to vacuum pumping means; placing said sphere inside an oven;evacuating the system of tubes and sphere; heating said sphEre to removegases from the walls and then allowing the sphere to cool; moving saidampoule containing the wax inside the necked-down tube to a positionnear the sphere; heating the sphere to a temperature sufficient to meltthe wax without degrading it so that the wax coats the inside of thesphere; removing the sphere and tubes from the oven; moving saidwax-containing ampoule back and out of said neckeddown tube; moving saidalkali-metal-containing ampoule into said neckeddown tube; heating thenecked-down tube at the end away from the end communicating with thesphere to seal it off and breaking off the second and third tubestherefrom after the seal; and breaking the frangible tips of theremaining ampoules.
 2. The method of claim 1, wherein the alkali metalis cesium.
 3. The method of claim 1, wherein the getter comprises a filminside its containing ampoule.
 4. The method of claim 1, wherein thegetter comprises a film of barium inside its containing ampoule.
 5. Themethod of claim 1, wherein the getter-containing andalkali-metal-containing ampoules are made of Pyrex glass.
 6. The methodof claim 1, wherein the ampoules and the tubes are made of Pyrex glass.